This invention relates to the application of laser light properties to a diffraction edge remote position sensor for detecting the position of a surface or interface, and in particular, but not exclusively, to such a sensor for determining the position of the vertex of a partially fabricated contact lens during the manufacture thereof.
In one known method of manufacture of contact lenses, for example, that disclosed in U.S. Pat. No. 4,455,901, a cylindrical blank of plastics material is machined on a lathe to form the inner concave surface of the lens to be produced. Manufacture of the lens is completed by attaching the machined concave surface to a domed metal arbour by means of wax or other suitable meltable material, mounting the metal arbour in the chuck of the lathe and machining the exposed material to form a convex surface defining the outer surface of the lens and then removing the lens from the arbour.
It will be appreciated that, in order to ensure that the lens is neither too thick nor too thin, it is necessary to be able to determine the vertex position--that is, the point of intersection of the axis of symmetry of the lens with the inner surface--with a high degree of accuracy. If the lens is too thick or too thin it will need to be scrapped because it will not perform to its optical design specifications. At present, the position of the vertex of the lens is determined by using a micrometer to measure the distance between the vertex of the concave surface and the plain end of the blank, and then marking this dimension on the arbour. Knowing this dimension, the blank is machined to form the concave surface defining the outer surface of the lens at a distance to provide a lens of the required thickness. This process is, however, time consuming and does not provide sufficient accuracy for production of thin lenses. In the particular manufacturing technique studied, it was necessary to be able to determine the position of the vertex with an accuracy in the region of .+-.10.0 microns.
A need therefore exists for a sensor capable of determining the vertex position of the formed surface of a partially fabricated contact lens with sufficient accuracy to enable the other surface to be formed to produce a contact lens having an acceptable thickness. Diffraction phenomena generated by laser sources have previously been used for demonstrations of the wave nature of light. Such applications of diffraction have been mostly limited to image processing via spatial filtering and have not been previously used for determining the vertex position of a partially fabricated contact lens to within the required accuracy. However, in the parent application, the present inventor recognized that the properties of laser sources, namely, the high beam intensity and sub-millimeter beam diameter, are ideally suited for application to edge and position sensing. The present invention relates to further enhancements of the method therein disclosed.